1. Field of the Invention
The present invention relates to holographic disc recording systems, and more specifically relates to a holographic disc recording system for recording information stored on a transmissive source disc onto a target disc composed of a holographic recording material in the form of holograms.
2. Description of the Related Art
Holographic recording techniques for recording information on a holographic storage medium in the form of holograms generally involve recording a plurality of holograms at a single recording location in the holographic storage medium by multiplexing the holograms. Holograms used for recording information are sometimes referred to as data pages. Thus, to multiplex a plurality of holograms means to multiplex a plurality of data pages. When a plurality of data pages are multiplexed, one of the characteristics of a reference beam used for recording is changed for each data page. For example, the angle at which the reference beam is incident on the storage medium or a wavelength of the reference beam is changed for each data page.
A spatial light modulator (SLM) constructed of a liquid crystal display or the like is typically used to imprint the information to be recorded on a signal beam. When a desired multiplexed hologram is to be read out, a reference beam having the same characteristic as that used for recording that hologram is used for reproducing it, and then the data page of the reproduced hologram is read with a two-dimensional detector array.
However, the above-described known holographic recording technique has several disadvantages. That is, the recording density of each data page is limited by the minimum pixel size and the minimum pixel pitch of the SLM and the two-dimensional detector array. Therefore, an extremely large number of holograms must be recorded at each of the recording locations in order to achieve high recording density. Depending on the method of multiplexing used and the amount of overlap between the adjacent recording locations in the medium, the shape of the volume occupied by a single hologram will vary, as will the effective number of holograms which can be multiplexed and recorded at a single recording location in the storage medium. Both of these factors may cause variations in reproduction intensity between the holograms recorded on the storage medium and between the pixels in each hologram. Such variations in reproduction intensity may lead to data readout errors, and complex reproduction intensity compensation techniques are required to reduce the probability of the data readout errors to a predetermined level. In addition, if the recording locations are arranged such that they do not overlap each other on the storage medium, there is a problem in that the dynamic range is reduced since the recording material between the adjacent recording locations cannot be used.
Furthermore, there is currently no feasible method for fast replication of a holographic memory obtained by using the above-described holographic recording technique. In order to copy such a holographic memory onto a target disc, a process of sequentially recording a plurality of holograms to be multiplexed must be performed at each of the recording locations in the target disc. In addition, in order to reduce the variations in reproduction intensity between the holograms multiplexed at the same recording location in the target disc, a complex exposure process must be repeated at each of the recording locations.